FUTURE GENERATION TECHNOLOGY STATUS WITHIN THE HIGH- PERFORMANCE PV PROJECT

Martha Symko-Davies, Robert McConnell and Fannie Posey-Eddy

National Center for National Renewable Energy Laboratory

2006 IEEE 4th World Conference May 11, 2006

NREL/PR-520-39867 Presented at the 2006 IEEE 4th World Conference on Photovoltaic Energy Conversion (WCPEC-4) held May 7-12, 2006 in Waikoloa, Hawaii. Disclaimer and Government License

This work has been authored by Midwest Research Institute (MRI) under Contract No. DE-AC36-99GO10337 with the U.S. Department of Energy (the “DOE”). The United States Government (the “Government”) retains and the publisher, by accepting the work for publication, acknowledges that the Government retains a non-exclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for Government purposes.

Neither MRI, the DOE, the Government, nor any other agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe any privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not constitute or imply its endorsement, recommendation, or favoring by the Government or any agency thereof. The views and opinions of the authors and/or presenters expressed herein do not necessarily state or reflect those of MRI, the DOE, the Government, or any agency thereof. High Performance PV Project Description

Explore the ultimate performance limits of PV technologies, doubling sunlight-to-electricity conversion efficiencies, lowering costs

Bring thin-film tandem cells and modules toward 25% and 20% efficiencies, respectively.

Develop multijunction concentrators able to convert more than one- third of the sun’s energy to electricity (i.e., 33% efficiency)

Increase baseline efficiency of 3rd generation PV concepts (from 39% baseline to 50%) and increase efficiency of potentially low-cost organic solar cells (from 11% baseline for the dye cell to 20%).

Enable progress of high efficiency technologies toward commercial- prototype products Efficiency (%) 40 12 16 20 28 32 36 24 1975 8 4 0 Masushita RCA Thin FilmTechnologies Crystalline SiCells Multijunction Concentrators Emerging PV Emerging Multijunction polycrystalline (various technologies) Si Nano-, micro-,poly- Si:H(stabilized)Amorphous CdTe Cu(In,Ga)Se Thin Si Multicrystalline Single crystal Two-junction (2-terminal,monolithic) monolithic) Three-junction (2-terminal, Organic cells Dye cells University of Maine RCA Monosolar RCA 2 Best Research-Cell Efficiencies 1980 RCA Boeing Boeing Kodak RCA Westing- tt University State house No. Carolina RCA Kodak RCA Boeing ARCO Solarex 1985 Spire Solarex ARCO Stanford AMETEK Varian Boeing So. Florida 1990 Photon Energy Photon University UNSW Spire Georgia Tech Georgia EPFL Boeing United Solar United NREL UNSW Sharp Euro-CIS 1995 NREL UNSW Energy Japan Georgia Tech Georgia NREL UNSW (2µm onglass) Kaneka EPFL UNSW (small area) (small AstroPower United Solar United UNSW Spectrolab NREL NREL/ NREL 2000 Spectrolab 14x concentration University Linz University NREL Cu(In,Ga)Se NREL NREL Spectrolab (45µm thin-film (45µm NREL Boeing- Stuttgart transfer) Univ. Groningen 2 NREL University Linz University Linz University (CdTe/CIS) FhG-ISE NREL 026587193 (large area) (large NREL Siemens (inverted, (inverted, matched) 2005 Konarka NREL Sharp Sharp NREL mis- Objectives of Future Generation

• Increase baseline efficiency of 3rd generation PV concepts (from 39% baseline to 50%) and increase efficiency of potentially low- cost organic solar cells (from 11% baseline for the dye cell to 20%). • Support the DOE Technologies Program: Multi- Year Technical Plan (MYTP) – Organic Solar Cells • Initiate theoretical studies for doping organic materials • Determine operational characteristics of excitonic solar cells using biomimetic, organic, and concepts • Assess efficiency potential, stability, and reliability of organic polymer and small-molecule solar cells • Identify commercialization pathways for promising new technologies via university/industrial partnerships – Dye-sensitized Solar Cells • Assess efficiency potential, stability, and reliability of inorganic/organic solar cells • Assess dye-sensitized options involving solid-state electrolytes. Objectives (continued)

• Support the MYTP – Nanostructure Solar Cells • Determine operational characteristics of excitonic solar cells using nanotechnology concepts---including biomimetic concepts mimicking a solar biological process – Third-Generation Technologies • Demonstrate feasibility of third-generation PV devices such as hot-carrier and impact-ionization concepts • Assess potential of for achieving third- generation goals of very high efficiency and very low cost. • Future Generation--Subcontractors – Organic Solar Cells • Princeton University (Steven Forrest) – Project Title: Approaching 10% Conversion Efficiency using Tandem Organic Photovoltaic Cells with Enhanced Optical Coupling – Collaborator: Global Photonic Energy Corp. • Northwestern University (Thomas Mason) – Project Title: Interface and Electrode Engineering for Next-Generation Organic Photovoltaic Cells – Collaborators: BP Solar, Konarka, Northwestern Departments of Chemistry, Physics and Astronomy, and Materials Science and Engineering. – Dye-Sensitized Solar Cells • University of Colorado (Josef Michl) – Project Title: Ultra High Efficiency Excitonic Solar Cell – Collaborators: University of Nevada-Reno, University of Wisconsin-Madison, Northwestern University, NREL (Art Nozik). – Third-Generation Technologies • University of Delaware (Christiana Honsberg) – Project Title: Theoretical and Experimental Investigation of Approaches to >50% Efficient Solar Cells – Collaborators: Imperial College, Purdue University, University of New South Wales. Summary of Activities

• Methods and Expected Outcomes--Subcontractors – Organic Solar Cells • Princeton University (Stephen Forrest) – Develop “toolbox” of concepts for optimizing organic solar cells targeting 10% efficiency • Northwestern University (Thomas Mason) – Characterize stability of n-type TCOs for candidate organic solar cells – Dye-Sensitized Solar Cells • University of Colorado (Josef Michl) – Synthesize multiple-exciton generation sensitizer for dye solar cell targeting 20% efficiency – Third-Generation Technologies • University of Delaware (Christiana Honsberg) – Develop design rules for non-conventional PV conversion processes leading to >50% efficient solar cells In-house Activities

• NREL Basic Sciences Center (High Efficiency Concepts PV) – Third Generation Technologies • Third Generation PV: Quantum Dot Solar Cells (Art Nozik) – Dye-Sensitized Solar Cells • Dye-Sensitized Solar Cell Research (Art Frank) – Computational Modeling, Analysis and Design of PV Materials • TCO, III-V, Polycrystalline thin films and Si (Suhuai Wei) – Solid State Spectroscopy of PV Materials • Enhance designs for high efficiency monolithic multi-bandgap tandem solar cells. Modulated Reflectance (MR) measurements, and Raman measurements(Angelo Mascarenhas) – Solid State Theory of PV Materials and Devices • Polycrystalline thin films, new materials for wide-bandgap thin-film top cells, enhance efficiencies of III-V tandem cells for high performance by inserting in them particles creating Intermediate Gap absorption (Alex Zunger) Exciton Solar Cells

• Electrons and holes are still created Dye Solar Cell simultaneously • But they are bound together and move as bound pairs to the nearest material boundary • Separation of electrons and holes at a material interface is followed by diffusion or electrochemical drivers of the charge carriers • Primarily associated with organic materials……plastics • Potentially low costs for materials and processing Multiple Electron-Hole Pairs-per-Photon Solar Cells

• Electron and hole created simultaneously • Hot electron and hole create multiple (m) electron-hole pairs through Auger emission and absorption instead of thermalizing • Need materials (e.g., quantum dots) susceptible to creation of multiple Auger electron-hole pairs • Recently observed by NREL and Los Alamos scientists…3 carriers for 1 photon • Can be added to a multijunction solar cell stack to achieve higher efficiency

H. Queisser et al., Max Planck Inst., Solar Energy Materials and Solar Cells, 41/42, 1996 Intermediate-Band Solar Cells

• Involves a narrow energy level, instead of stacks of energy bands as in multijunction cells • Theoretical efficiency advantages over equivalent multijunction cells • Can simplify and augment multijunction solar cell efficiencies

A. Luque et al., U. Politecnica de Madrid Band diagram of a solar cell Phys. Rev. Lett., 78, No. 26, 1997 with an intermediate band. PI: Stephen Forrest, Princeton University

• Research Objectives: • Significant Results: Demo organic PV cells with high • In a hybrid cell consisting of the organics CuPc/C60 efficiency by using new structures based (material system first demonstrated in this program), on vapor deposited small molecular record efficiencies of 5.9% under 1 sun, AM1.5 weight organic thin films. illumination observed. • First demonstration of organic bulk heterojunction • Approach: based on small molecular weight materials 1. Demonstrate new materials systems • Formation of a bulk heterojunction by and structures to enhance efficiency. thermal annealing of small • CuPc/C60 demonstrated molecules: • Double heterstructure cell invented 2. Use tandem structures with metal nanoparticles for plasmon optical field enhancements

• Industry Impact: Currently working with Global Photonic Energy Corp. to find partners for production and commercialization of our 5+% efficiency cells on low cost plastic (flexible) substrates. Peumans, et al., Nature 425, 158 (2003) Controlled Growth of a Bulk Heterojunction Ideal Bulk HJ PV Cell

Tcell Organic Vapor Phase Deposition: Different strain and growth conditions ⇒ different structure

Bulk HJ Cell

Donor Layer: Highly Folded , 39 (2005) Conventional Cell 4

Acceptor Layer: Planarizes Cell Mater., al. Nature et F. Yang, Some Significant Issues Addressed

√ • Scalability – Cells thin to maximize absorption and minimize resistance/exciton bottleneck limits ⇒ small area to avoid shorts – ITO very resistive • Low V ⇒ new materials & concepts √ oc • Organics typically do not absorb into the IR • Reliability ⇒ packaging, new materials • Large area, low cost manufacturing – Plastic substrates √ –OVPD PI: Thomas O. Mason, Northwestern University

• Research Objectives: • Significant Results: To advance the science and technology • Interfacial science of OLEDs applicable to OPVs of next-generation, efficient, easily • Theory-guided synthesis of novel TCOs manufacturable, and durable organic photovoltaics. • Correlation of surface vs. bulk TCO defect/electronic properties • Approach: • Nanostructured TCO electrode materials 1. Improved transparent electrodes Proposed Organic Photovoltaic of the (transparent conducting oxides, TCOs) Future for efficient organic photovoltaics 2. Engineered organic-transparent 1. New TCO materials electrode interfaces for high-efficiency Hole- as alternative anodes Blocking organic photovoltaics - - - - 2. Charge-blocking Layer, HBL at interfaces - - Cathode Light • Industry Impact: 3. Crosslinked Demonstration of prototype OPVs and Anode multilayer structure + + technology transfer of novel transparent Electron- 4. Interfacial electrodes and engineered organic-TCO Blocking + + + + stabilizers Layer, EBL interfaces to NREL and our partners (BP Photoactive Solar, Konarka Technologies) Layer PI: Josef Michl, CU Boulder

2 e’s produced e- instead of one e- e- 300 Å TiO2 e- hν Vph Singlet fission e- e- I -/I- TCO molecule 3 electrode 3.2 eV h+ SF molecule 50 Å

TiO2 electrolyte

Device efficiencies could Transparent Pt increase by ~ 50% with SF TCO Counter Electrode Electrode PI: Arthur J. Nozik, NREL (Collaborative Project with Office of Science/BES/Chemical Sciences) • Research Objective: • Significant Results: To demonstrate multiple electron-hole pair 1. Quantum yields of up to 300% (3 electron-hole pairs (exciton) generation from single photons in per photon) were measured in PbSe and PbS quantum dots (QDs) and apply this effect to QD solar cells to produce greatly enhanced quantum dots at photon energies 4 times the QD photocurrent and conversion efficiency bandgap. 2. The enhanced performance and increased efficiency of QD solar cells based on MEG have been calculated. •Approach: • Synthesize QDs that are expected to show greatly enhanced multiple exciton generation • Graphics: (MEG). 300 • Measure MEG using transient absorption (b) spectroscopy.

• Develop and characterize nanocrystalline solar Eg (Homo - Lumo) cells sensitized with QDs exhibiting efficient MEG 0.72 eV 250 and greater photocurrent. Quantum 0.72 eV • Model performance of QD solar cells showing 0.72 eV Yield 0.82 eV efficient MEG. 0.91 eV Industry Impact: 200 0.91 eV • 0.91 eV Ultra-high conversion efficiency in 3rd-generation 0.91 eV solar cells could significantly reduce the cost of PV PbS - 0.85 eV power. Incorporating quantum dot solar cells into (%) Quantum Yield 150 multijunction cells already in production could provide a feasible route to 50% efficient solar cells. Quantum yield vs. photon energy (as ratio of QD bandgap) for PbSe and PbS QDs of different bandgaps (i.e., QD diameters). (Published in: NanoLetters, 5, 5, 865-871, May, 2005) PI: Christiana Honsberg, University of Delaware Project Objectives: Develop realistic solar cell device designs capable of reaching an efficiency of 50% Approach: – Analyze physical concepts that increase efficiency above single- junction device • Multiple energy level solar cells achieve this and may be implemented with wide range of materials – Develop practical models for quantum dot multiple energy level solar cells: 1. Program to calculate band structure based on upon energy levels derived from Schrödinger Equation. Searches space of III-V alloys for material that synthesize an optimum band structure. 2. Program to determine optimum band structure taking into account non-idealities including: • Concentration • Finite intermediate band width • Spectral selectivity • Intraband transitions within the intermediate band Future Directions 1. Calculate and include in efficiency calculations the impact of a finite density of states on QD MEL solar cells on radiative transitions (absorption and recombination) • Calculations based on piece-wise

continuous parabolic density CB1 of states (DOS). E=hfa E E=hfc 2. Refine band structure model as G,CI needed to include additional effects Energy IB2 ΔIB (e.g. strain) and determine optimum IB1 E E=hfb materials including finite DOS G,CI VB calculations. 1

3. Experimentally implement QD MEL Wave vector solar cells with optimum materials and demonstrate validity of model and design. The Promise of Future Generation PV Remains

• Theoretical efficiencies are much higher than demonstrated efficiencies. • New PV materials and devices may be cheaper and more plentiful. • An English tradition says that if you follow a rainbow to its end, you will find a pot of gold. But A cautionary note comes from to find gold, you have to be the ancient Greek writer patient, supportive of a variety Sophocles who said: “Look and of approaches and capable of you will find it—what is affording risk. unsought will go undetected.”